Such pulse generators are used in many applications in the field of process automation with reference to its applications of ultrasonics and radar.
Thus, the ultrasonic flow measuring devices permit, in a simple manner, contact-less determination of volume flow rate in a pipeline.
The known ultrasonic flow measuring devices work either on the basis of the Doppler principle or the travel time difference principle.
In the case of the travel time difference principle, the different travel times of ultrasonic pulses are evaluated relative to the flow direction of the liquid.
For this purpose, ultrasonic pulses are transmitted both in the direction of flow and opposite thereto. From the travel time difference, the flow velocity can be determined, and, with known diameter of the pipeline cross section, the volume flow rate.
In the case of the Doppler principle, ultrasonic waves of known frequency are coupled into the liquid, and the ultrasonic waves reflected by the liquid are evaluated. From the frequency shift between the waves coupled in, and those reflected, one can likewise determine the flow velocity of the liquid.
Reflections in the liquid occur, however, only when air bubbles or impurities are present therein, so that this principle finds use mainly in the case of contaminated liquids.
The ultrasonic waves are produced, or received, as the case may be, with the help of so-called ultrasonic transducers. For this purpose, ultrasonic transducers are placed securely on the pipe wall of the pipeline section of concern. More recently, clamp-on ultrasonic measuring systems are also obtainable. In the case of these systems, the ultrasonic transducers are held against the tube wall using only a clamp fastener. Such systems are known e.g. from EP-B-686 255, and from U.S. Pat. Nos. 4,484,478 and 4,598,593.
Another ultrasonic flow measuring device, which works on the basis of the travel time difference, is known from U.S. Pat. No. 5,052,230. The travel time in this case is determined using bursts, i.e. short ultrasonic pulses.
The ultrasonic transducers are usually made of a piezoelement and a coupling wedge. Ultrasonic waves are produced in the piezoelement and guided by way of the coupling wedge to the pipe wall, and, from there, into the liquid. Since the sound velocities in liquids and plastics are different, the ultrasonic waves are refracted at the transition from one medium to the other. The refraction angle is determined by Snell's law. The refraction angle is, consequently, dependent on the ratio of the propagation velocities in the two media.
Often, ultrasonic flow measuring devices are used in explosion-protected areas. In these areas, ignitable gases are present, whose ignition is to be avoided. For explosion-protected areas, there are corresponding safety specifications, in order to prevent endangerment of plant and persons. An ignition of the gases can happen, when certain values of outwardly-acting voltage, current, inductance or capacitance are exceeded and, consequently, sufficient energy is introduced into the gas, that an ignition process is triggered. High pulse-voltages are needed to get a sufficient measurement accuracy. With small pulse-voltages, signal evaluation becomes extremely difficult. Ex-areas are divided in the known safety standard into different zones, which are governed by different safety criteria. In the case of malfunctions of the control logic that governs the production of the voltage pulses, it is not out of the question that the number of the pulse periods per burst will become too large, the pulse frequency too high, or the burst repetition rate too high. All of these cases can lead to a gas ignition. The situation, wherein the voltage of the voltage supply unit is always at the output, must likewise be prevented.
An object of the present invention is, therefore, to provide a pulse generator suited especially for ultrasonic flow measuring devices and also permitting a safe application in Ex-areas.
This object is achieved by a pulse generator with at least one capacitor between the control logic and the switch, which decreases the pulse voltage when the pulse frequency of the control pulses is too small, and between the voltage control unit and the switch, an RC-member is arranged, which decreases the pulse voltages, when the repetition rate of the control pulses is to high.